There has recently been developed a battery operated wristwatch of solid state components programmed to continuously keep the time. However, the watch uses an electronic display which, to conserve power, is operated only upon signal rather than continuously. The display may be of the light emitting diode (LED) type or liquid crystal display (LCD) type and has been most frequently operated on demand as a result of manually closing a switch.
There have also been several proposals for using demand switches which do not require depression or actuation by the wearer's free hand.
As one example, U.S. Pat. No. 3,871,170 discloses use of a switch actuated in response to inertia resulting from a deliberate arm movement to move a conductive ball of mercury rapidly between two positions to close two sets of contacts in rapid succession. This type of switch requires a quick unnatural motion of the arm which may frequently be inconvenient and uncomfortable to the wearer.
In U.S. Pat. No. 3,841,080 there is disclosed a tilt sensitive switch wherein an asymmetrical balanced disc is used to actuate electrical contacts for causing display on demand. U.S. Pat. No. 3,921,385 uses a round conductor or ball that is actuated in a cavity by gravity to close on demand a switch actuating the wristwatch display. The mechanical switches of these latter two patents require substantial space and may be subject to undue wear as well as undesirable noise in operation. Both of these patents apparently recognize the superior features that mercury tilt switches would offer as demand switches for wristwatches. However, both patents conclude that known merucy tilt switches also present problems which lead the patentees to reject them for such use.
In my copending U.S. pat. application Ser. No. 600,922, for "Mercury Tilt Switch", filed July 31, 1975, I have disclosed a tilt switch suitable for actuating an LED display or the like for a battery-operated wristwatch. The switch of that application is of a wet construction, employing merucy-wettable surfaces.
My present invention is directrd to a new and improved mercury-type tilt switch that is particularly suitable, both as to size and operating characteristics, as a demand switch for a battery-operated wristwatch and to a wristwatch utilizing such switch.
An example of a prior art mercury tilt switch is disclosed in U.S. Pat. No. 3,599,745, issued to Benjamin F. Hughes. This switch comprises a housing having a globule of mercury therein, which housing is attached to a tiltable vehicle. The housing is of insulating material with a hemispherical top and a base at the center of which an elevated table is formed with a concave seat into which a pair of electrical contacts extend. The switch can be manipulated to locate the mercury globule on the concave seat to bridge the contacts. If the switch is tilted more than a predetermined extent from the horizontal, as by tilting of the vehicle on which it is mounted, the globule is displaced from the seat by gravity and falls into an annular groove surrounding the seat. The table on which the seat is located being raised and having concave ends, mere return of the switch in the reverse path to its opening movement through the angle of tilt, as by the vehicle being returned to a level position, will not cause the globule to return to the seat. Instead, the switch must be manipulated, as by more or less inverting it, to center the globule over the seat and thus reset it to bridge the contacts.
Another tilt type switch is disclosed in U.S. Pat. No. 3,351,726 issued to Leonard W. Cook. This switch includes a hollow gas-tight cylindrical metal housing with a terminal rod extending in insulated relationship into the housing through a member of inorganic insulating material at an end of the cylinder. A quantity of liquid metal of high surface tension, such as pure mercury, is disposed in the casing. The mercury is of a size large enough to bridge the terminal rod and the side wall of the housing when the switch is in an upright position and to separate from the terminal rod and engage only the housing when tilted at what the drawing reveals as a very large angle from the upright position.